Repulpable pressure-sensitive adhesive compositions

ABSTRACT

A water-dispersible, inherently tacky pressure-sensitive adhesive comprising a tacky emulsion polymer formed of from about 80% to 90% by weight of monomers comprising a major portion of at least alkyl acrylate containing from 4 to about 8 carbon atoms in the alkyl group and from about 20% to 10% by weight of a mixture of acrylic acid with at least an oleophilic unsaturated carboxylic acid, said emulsion polymer formed in the presence of anionic and nonionic surfactants to provide a polymer having a glass transition temperature of -15° to -50° C. A sufficient amount of chain transfer agent is employed to provide an emulsion polymer which when coated on a repulpable paper substrate enables recovery of paper fibers substantially free of adhesive under conditions of TAPPI Useful Method 213 and/or 204, and resistant to loss of adhesive properties on exposure to high humidity.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 08/211,763 filed Apr. 14,1994, which is a continuation-in-part of application Ser. No. 799,555filed Nov. 27, 1991, now abandoned, which is a continuation-in-part ofapplication Ser. No. 777,667 filed Oct. 15, 1991, now U.S. Pat. No. b5,196,504.

BACKGROUND OF THE INVENTION

The present invention relates to water- or alkali-dispersible emulsionpolymer systems which are functional as pressure-sensitive adhesives.The adhesives of the instant invention provide an ecologically friendlyreplacement for pressure-sensitive adhesives which hinder clean paperrecovery during repulping.

For a label or tape constructions to be repulpable, the adhesive must,for some markets, be water dispersible or cold water soluble and, forsuch markets, pass Tappi Useful Method 213 (Tappi UM 213) "Repulpabilityof Splice Adhesive Compositions," incorporated herein by reference.

Other markets which utilize hot water or alkali for repulping accept adifferent test. A product which passes a Tappi Useful Method 204 (TappiUM 204), a de-inking and repulping test incorporated herein by referencewill also satisfy requirements for elevated temperature alkalirepulping.

Most rubber-based and acrylic adhesive systems are not repulpable.During the repulping process (fiber shearing), the adhesives tend toagglomerate and form globules, commonly referred to in the trade as"stickies." Stickies adversely affect paper quality and have a negativeimpact on the process of recycling paper.

Several approaches can be taken to remove adhesive stickies. One hasbeen to use water-dispersible adhesive microspheres prepared byprocesses described in U.S. Pat. Nos. 3,691,140 to Silver, 4,155,152 toBaker et al., 4,495,318 and 4,598,212 to Howard, and 4,810,763 to Mallyaet al., each incorporated herein by reference, and applied atappropriately low coat weights. While the adhesives disperse in thepaper fiber adequately during the repulping process, the adhesive is notremoved from the paper fibers. Rather, the adhesive microspheresaccumulate in the paper fibers, reducing paper quality and limit thenumber of times the paper fibers can be repulped or recycled.

Water-soluble adhesive systems which provide for complete adhesiveremoval from the fiber have been used commercially. Typically,commercial water-soluble products have suffered from (a) poor shelf life(the tape or label is sold in a sealed plastic bag); (b) poor humidityand/or heat-aged performance; and (c) a propensity to bleed into thepaper facestock (which reduces peel performance). Each of these problemsis related to the high water-sensitivity of the respective products.Such water-soluble adhesives and tapes are described, for instance, inU.S. Pat. No. 3,441,430 to Peterson; U.S. Pat. No. 3,865,770 to Blake,U.S. Pat. No. 4,052,368 to Larson, U.S. Pat. No. 4,413,080 to Blake, andU.S. Pat. No. 4,569,960 also to Blake, each incorporated herein byreference.

There is a need therefore to provide adhesives for a general purposelabel and tape construction which are repulpable and not sensitive tochanges in relative humidity and where no special precautions arerequired for storage or use as an adhesive for a label or tape.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an adhesive systemwhich comprises inherently tacky water-dispersible pressure-sensitiveadhesives formed by emulsion polymerization of from about 55% to about90% by weight of nonacid monomers, predominantly alkyl acrylates,interpolymerized with from about 10% to about 20% of a mixture ofacrylic acid and at least one oleophilic unsaturated carboxylic acid,preferably a mixture of acrylic acid with methacrylic acid, with someportion of the polymerization occurring in the presence of a chaintransfer agent. The repulpable pressure-sensitive adhesives have a glasstransition temperature in the range of about -15° to -50° C. and arepreferably formed in the presence of a mixture of nonionic and anionicsurfactants.

Useful repulpable pressure-sensitive adhesive polymers may be preparedby one stage of emulsion polymerization, sequential polymerization or byblending separately formed emulsion polymers. Sequential emulsionpolymerization is preferred. However formed, the emulsionpressure-sensitive adhesives may be blended with other adhesive polymersto control bleed without loss of repulpable properties as set by TappiUM 204. Tackifiers can be used to enhance adhesion to substrates such ascardboard. External or internal crosslinking of the polymer can be usedto enhance cohesive strength and reduce bleed into porous papersubstrates. Pressure-sensitive compositions contemplated to be formed inaccordance with the instant invention pass TAPPI UM 204. Adhesives orblends of adhesives may also pass Tappi UM 213.

The presently preferred pressure-sensitive adhesive compositions arebased on a copolymer of 2-ethylhexyl acrylate, methyl acrylate, vinylacetate, methacrylic acid and acrylic acid in which the acid content isfrom about 10% to about 20% by weight of the polymer and acrylic acidcomprises from about 30% to about 90% by weight of the total unsaturatedcarboxylic acids, and formed by sequential polymerization in which about40% to 80% of the monomers are polymerized in a first stage, followed byaddition of the balance of the monomers. Sequential polymerizationappears to provide the best balance of adhesive properties,dispersibility and humidity resistance.

Some portion of the preferred adhesive polymers used as a repulpableproduct is polymerized in the presence of from about 0.5% to about 1.5%by weight of the monomers of a chain transfer agent, preferablyn-dodecyl mercaptan. There may be optionally added an internalcrosslinker during polymerization or, after polymerization, an externalcrosslinker to enhance cohesive strength and reduce paper bleed.

There may also be added to reduce bleed certain amounts of otheremulsion polymers which enable the adhesive to pass Tappi UM 213 and/orTappi UM 204.

The preferred repulpable pressure-sensitive adhesive polymers containabout 55% to about 90% by weight of an alkyl acrylate containing from 4to about 8 carbon atoms in the alkyl group, from about 1% to about 15%by weight methacrylic acid, about 5% to about 19% by weight acrylicacid, from 0% to about 15% by weight vinyl acetate, and from 0% to about15% by weight methyl acrylate formed in the presence of 0% to 1.5% byweight of the monomers of n-dodecyl mercaptan and from about 0% to 5% ofthe weight of the monomers of aluminum acetate as the externalcrosslinker and/or an internal crosslinker.

The presently preferred repulpable pressure-sensitive adhesivecomposition is a mixture of about 85.5% by weight of a sequentiallypolymerized polymer system containing about 60% by weight 2-ethylhexylacrylate, about 12.5% by weight vinyl acetate, about 12.5% by weightmethyl acrylate, about 13.5% by weight acrylic acid and about 1.5% byweight methacrylic acid, blended with about 4.5% by weight of acopolymer containing about 56.8% 2-ethylhexyl acrylate, about 37.8% byweight butyl acrylate about 1.9% by weight acrylic acid, about 3.8% byweight methacrylic acid, and about 0.6% by weight with itaconic acid,tackified with about 10 parts by weight of a rosin ester.

The adhesive is coated, typically to a coat weight of 20-25 g/m² andapplied as a continuous coat on a label or tape backing or face stock.Such a product has the property of being re-emulsified or dispersedunder the action of water, with or without agitation.

Monomers such as diallyl maleate formed in the absence of a chaintransfer agent such as n-dodecyl mercaptan may also be used as such ifthey pass Tappi UM 213 and/or UM 204 or if when added to an adhesiveformed in the presence of n-dodecyl mercaptan will pass Tappi UM 213and/or UM 204.

DETAILED DESCRIPTION

The present invention relates to water-dispersible inherently tackypressure-sensitive adhesive polymers for repulpable paper label and tapeapplications. When employed with repulpable paper label and tape stock,the adhesives have the capability of being dispersed in cold waterand/or hot alkali solution (88° C. or more) and enable recovery of paperfiber free of contamination by the adhesive.

The pressure-sensitive adhesives useful in the instant invention areprepared by batch (single stage) or sequential polymerization or byblending of batch polymers. Glass transition temperature as measured byDSC (Differential Scanning Colorimetry) of the final polymer or blend ofpolymers is in the range from about -15° C. to -50° C. Repulpabilityrequires limiting molecular weight of some amount of the polymer. Thismay be accomplished by utilizing as the adhesive polymer or a componentof a polymer blend, an adhesive polymer formed in the presence of achain transfer agent, preferably n-dodecyl mercaptan, at a concentrationproviding a net polymer blend having a good balance of dispersibility,pressure-sensitive adhesive performance, humidity resistance and aresistance to bleed.

The emulsion formed pressure-sensitive adhesives of the instantinvention contain, on a percent by weight basis, from about 55 to about90% by weight total of one or more alkyl acrylates containing from 4 toabout 8 carbon atoms in the alkyl group. Useful alkyl acrylates includen-butyl acrylate, 2-ethylhexyl acrylate, iso-octyl acrylate, and thelike, as well as mixtures thereof.

Other modifying monomers may be effectively employed so long as theglass transition temperature of the final adhesive remains in the rangeof from about -15° C. to -50° C. Representative modifying monomersinclude lower alkyl acrylate such as a methyl acrylate; vinyl esters,such as vinyl acetate, vinyl butyrate, vinyl propionate, vinylisobutyrate, vinyl valerate, vinyl versitate, and the like; and diestersof an unsaturated dicarboxylic acid and mixtures thereof, such asdi-2-ethyl hexyl maleate, di-2-ethyl hexyl fumarate, and dibutylfumarate and the like.

When employed, vinyl acetate and methyl acrylate improve the hydrophiliccharacteristic of the polymers, with methyl acrylate further acting toimprove cohesive strength. Vinyl acetate and methyl acrylate are eachemployed in a concentration of 0% to 15% by weight, and preferably inequal amounts.

Other preferred acrylate monomer combinations system employ a mixture of2-ethylhexyl acrylate and butyl acrylate with the preferred ratio of2-ethylhexyl acrylate to butyl acrylate being about 3 to 1.

The essential component of the repulpable polymers is a blend of acrylicacid with at least one unsaturated oleophilic carboxylic acid, such asmethacrylic acid and itaconic acid and the like, with a ratio of acrylicacid to oleophilic acid being from about 2:3 to about 7:1. The totalcarboxylic acid content of the polymer is from 10% to about 20% byweight. Blends of acrylic acid and methacrylic acid are preferred inwhich acrylic acid is present in a concentration of about 5% to about19% by weight of the polymer and methacrylic acid is employed in aConcentration of about 1% to about 15% weight of the polymer.

By the term "oleophilic" carboxylic acid as used herein there is meantunsaturated carboxylic acids which are more soluble in the monomeric oilphase of the emulsion than acrylic acid.

In all emulsion polymerization systems, monomers homopolymerize andcopolymerize. We have found that an oleophilic unsaturated carboxylicacid, such as methacrylic acid, is not only preferentially soluble inand copolymerizes with alkyl acrylates, but enhances the solubility ofacrylic acid in such monomers to promote copolymerization of acrylicacid with the alkyl acrylates.

When employed, butyl acrylate adds stiffness to the copolymer,methacrylic acid promotes acrylic acid copolymerization, and acrylicacid enhances adhesion to polar surfaces.

All or a portion of the emulsion pressure-sensitive adhesive polymersused in the instant invention are prepared in the presence of a chaintransfer agent present in an amount of from about 0.5% to about 1.5% byweight of the monomers, preferably from about 0.75% to about 1.25% byweight of the monomers. The preferred chain transfer agent is n-dodecylmercaptan or t-dodecyl mercaptan.

The repulpable emulsion pressure-sensitive adhesive polymers areprepared by providing an initial charge to a polymerization reactor ofan aqueous solution containing a nonionic surfactant which is preferablyan ethoxylated rosin acid emulsifier, most preferably an ethoxylatedrosin acid emulsifier containing about 46 ethylene oxide groups permolecule. There is added to the initial charge in the reactor a smallamount of the monomers separately prepared in a pre-emulsion containinganionic surfactant, preferably a sulfated ethoxylated nonylphenolcontaining about 6 ethylene oxide units per molecule and a free radicalinitiation system. The presently preferred nonionic surfactant is AR-150manufactured and sold by Hercules. The presently preferred anionicsurfactant is Alipal CO-436 manufactured by Rhone Poulenc. The amount ofnonionic surfactant employed may be from about 4% to about 8% by weightof the initial reactor charge, with anionic surfactant concentrationbeing from about 2.5% to about 5% by weight of the monomers in thepre-emulsion.

The reaction is redox initiated and the remaining pre-emulsifiedmonomers are added incrementally i.e., intermittently or continuouslyover a period of time.

While, as previously indicated, the monomers can be added in onepre-emulsion charge, it is preferred that the monomers be sequentiallyadded. It is presently preferred that from about 40% to about 80% of thetotal monomers be incrementally added to the initial charge andessentially totally reacted following by the incremental addition of theremainder of the monomers in a second pre-emulsion for reaction in thereactor. Excellent results have been obtained in sequentialpolymerization where the first charge of monomers contains about 15% to20% by weight carboxylic acid, and the second charge of monomerscontains about 10% to 25% by weight carboxylic acid. Independent of theamount of acid employed in any given charge, the net repulpable productshould contain about 10% to about 20% by weight polymerized carboxylicacids in which acrylic acid is present in an amount from about 30 toabout 90% by weight of the carboxylic acids.

We have found in general that the amount of chain transfer agentemployed provides a tradeoff between dispersibility, adhesiveperformance and humidity resistance, with the amount of chain transferagent employed being inversely proportional to the amount of acid in thecharge. It is believed the presence of high level of carboxylic acid inthe polymer enhances water-dispersibility, and therefore the polymer canhave a higher molecular weight. This allows a reduction in the amount ofchain transfer agent employed. At lower carboxylic acid concentrations,the polymer is less dispersible, and therefore more chain transfer agentis required to reduce molecular weight. As molecular weight is reduced,the tendency of the polymer to bleed into a porous paper facestock isincreased. This enhances, for some applications, the desirability ofadding to the monomers an internal crosslinker such as diallyl maleateor an external crosslinker such as aluminum acetate. With the externalcrosslinker, crosslinking occurs after polymerization. Crosslinkers maybe used in a concentration of up to about 1% by weight of the monomers,typically from about 0.2% to about 0.6% by weight of the monomers.

Bleed can also be controlled by adding modifying acrylate polymers of anacid content less than about 10% by weight. The presently preferredadditive polymers are externally prepared emulsion polymers. They areemployed in a concentration of 0% to about 40% by weight of the totalpolymers. One such polymer is a copolymer of 2-ethylhexyl acrylate andbutyl acrylate, containing about 2% by weight acrylic acid and about 3%by weight methacrylic acid with about 0.6% by weight itaconic acidformed by emulsion polymerization.

Water-dispersible pressure-sensitive adhesive polymers prepared byemulsion polymerization are described in detail herein. To besatisfactory, at least one polymer component of a composition must passa wash-off screening test. In this test, the adhesive is coated onto a 2mil polyester film at a coat weight of 20-25 g/m², dried in an oven at70° C. for 15 minutes, cooled to room temperature, and then held under awater tap to determine if the adhesive turns milky. If so then theadhesive has the ability to be dispersed or re-emulsified or washed froma surface. This may also be established by placing a sample in a beakerof water and allowing the sample to stand to determine if the water inthe beaker turns milky. A third screening test is to simply place dropsof eater on the adhesive surface and rub the water into the surface. Ifthe water turns .milky the adhesive is probably dispersible.

The following Examples and Controls are to illustrate the invention. Inrespect to Table IV, "E" means an Example representative of theinvention; "C" means Control; a formulation which did not pass criteriafor dispersibility; including a wash-off ("WO") screening test. Therequirement for ultimate re-pulpability means passing of TAPPI UM 213and/or UM204. A requirement for being useful as a pressure-sensitiveadhesive is passing, or at least marginally passing, a cardboardadhesion ("CA") test which is a manual looptack adhesion to recycledcardboard. In respect to Table IV "EHA" means 2-ethyl hexyl acrylate,"BA" means butyl acrylate, "VA" means vinyl acetate, "MA" means methylacrylate, "MAA" means methacrylic acid, "AA" means acrylate acid, "DAM"means diallyl maleate, "DDM" means n-dodecyl mercaptan. "A" means singleincremental addition polymerization, "S" means sequential, two stageincremental addition polymerization, and "B" means a blend of twoseparate "A" type polymers. With respect to an Example or Controlinvolving sequential polymerization or a blend ("a") means thecomposition of the first stage pre-emulsion monomer addition or thefirst polymer of a blend of polymers and ("b") means the composition ofthe second stage of preemulsion monomer additional or the second polymerof a blend. AR-150 is a nonionic ethoxylated rosin acid emulsifiermanufactured by Hercules and containing approximately 46 units ofethylene oxide per molecule. Alipal CO-436 is manufactured and sold byRhone Poulenc and is a sulfonated ethoxylated nonylphenol containing 6units of ethylene oxide per molecule. Triton X-100 is a octylphenoxypolyethoxyethanol nonionic surfactant having a HLB value of 13.5 andTriton X-165 is a nonionic octylphenoxy polyethoxyethanol having a HLBvalue of 15.8 both are manufactured by Union Carbide. Vinol (now AIRVOL203) is a polyvinyl alcohol protective colloid manufactured and sold byAir Products, Inc. In terms of performance, "F" means fail, "P" meanspass, and "M" means marginal. 50# EDP means electronic data alkalineprocessed paper at a 50 pound per ream weight. "WO" means wash-off inone or all combinations of three tests described above. "CA" CardboardLooptack Adhesion by the manual test. "OB" means an overnight bleed testto determine if at 50° C., the adhesive bleeds into and produce avisible stain on 50# EDP paper. "HH" means high humidity and again avisible stain on exposure to 90% humidity, at 25° C. for one week. Passin any bleed test means no visible stain. "Tappi" means TAPPI UM213and/or 204.

Examples 1-8 and Controls 1-5 are for compositions involvingsingle-stage polymerization where monomers in the weight percent shownwere polymerized in a single stage by addition of a pre-emulsioncontaining Alipal CO 436 as the surfactant in the concentration shown toa reactor containing an initial charge of water, AR-150 as thesurfactant, and ferric ethylenediaminetetraacetic acid and potassiumpersulfate in amounts shown below.

The presently preferred polymerization procedure involving sequentialaddition of monomers pre-emulsions and is specific for adhesive ofExamples E-10 as well as general to Examples E-9 to 11 and Controls C-6to C-9.

To a reaction vessel there was added materials shown in Table I.

                  TABLE I    ______________________________________           Component                   Grams    ______________________________________           WATER   150           AR-150  40           Fe EDTA.sup.1                   0.1    ______________________________________     .sup.1 Fe EDTA  Ferric salt of ethylenediaminetetraacetic acid.

The mixture was heated to 70° C. and treated with hydrogen peroxide forone half hour, then allowed to cool to 50° C. at which time there wasadded 3 grams K₂ S₂ O₈ and 9 grams of NaHCO₃.

There was separately formed, a pre-emulsion of the composition shown inTable II.

                  TABLE II    ______________________________________    Ingredient           % Monomers Grams    ______________________________________    Water                           255    Alipal CO-436 (60%)             40    K.sub.2 S.sub.2 O.sub.8         2.4    2EHA                 61         384    BA                   20.3       128    MAA                  12.2       76.8    AA                   6.5        41.2    Catalyst, tertiarybutylhydroperoxide(t-                                    7.2    BHP) Chain transfer agent, n-dodecyl    mercaptan (n-DDM)    ______________________________________

There is also formed a second pre-emulsion of the composition shown inTable III.

                  TABLE III    ______________________________________    Ingredient       % Monomers Grams    ______________________________________    Water                       75    Alipal CO436 (60%)          10    K.sub.2 S.sub.2 O.sub.8     0.6    2EHA             58.2       96    BA               29.1       48    MAA              8.8        14.6    AA               3.9        6.4    t-BHP                       0.6    n-DDM                       1.8    ______________________________________

50 grams of the first pre-emulsion was charged to the reactor at 50° C.and the balance incrementally added over a two hour period. Followingcompletion of addition of the first pre-emulsion there was started theincremental addition of the second pre-emulsion over a one hour period.The properties of the polymer formed are shown in Example 10 of TableIV.

                                      TABLE IV    __________________________________________________________________________    % BY WGT    E/C  EHA  BA  MAA  AA  DDM  MODE  % POLYMER    __________________________________________________________________________    E-1  60   20  12   8   0.50 A     100    E-2  60   20  12   8   1.00 A     100    E-3  60   20  12   8   1.50 A     100    C-1  60   30  6    4   0.00 A     100    C-2  60   30  6    4   0.25 A     100    E-4  60   30  6    4   0.25 A     100    E-5  60   30  6    4   0.50 A     100    E-6  60   30  6    4   0.50 A     100    E-7  60   20  8    12  1.50 A     100    E-8  60   30  4    6   0.50 A     100    C-3  60   30  6    4   0.50 A     100    C-4  60   30  6    4   0.25 A     100    C-5  60   30  6    4   0.50 A     100    E-9       (a)         61.0 20.3                  12.2 6.5 1.14 S     80       (b)         58.2 29.1                  8.8  3.9 1.09 S     20    E-10       (a)         61.0 20.3                  12.2 6.5 0.80 S     80       (b)         58.2 29.1                  8.8  3.9 0.76 S     20    C-6       (a)         61.0 20.3                  12.2 6.5 0.57 S     80       (b)         58.2 29.1                  8.8  3.9 0.54 S     20    C-7       (a)         61.0 20.3                  12.2 6.5 1.14 S     80       (b)         58.2 29.1                  8.8  3.9 1.09 S     20    E-11       (a)         61.0 20.3                  12.2 6.5 1.14 S     80       (b)         58.2 29.1                  8.8  3.9 1.09 S     20    C-8       (a)         61.0 20.3                  12.2 6.5 0.00 S     80       (b)         58.2 29.1                  8.8  3.9 0.00 S     20    C-9       (a)         61.0 20.3                  12.2 6.5 0.00 S     80       (b)         58.2 29.1                  8.8  3.9 2.18 S     20    C-10       (a)         60   20  12   8   1.00 B     50       (b)         60   30  6    4   0.25 B     50    E-12       (a)         60   20  12   8   1.00 B     85       (b)         60   30  6    4   0.25 B     15    E-13       (a)         60   20  12   8   1.00 B     90       (b)         60   30  6    4   0.25 B     10    E-14       (a)         60   20  12   8   1.00 B     95       (b)         60   30  6    4   0.25 B      5    __________________________________________________________________________                     PRE    INITIAL          EMULSION    TEST RESULTS    E/C  SURFACTANT                  %  SURFACTANT                              %  WO CA OB HH    __________________________________________________________________________    E-1  AR-150   5.00                     CO-436   3.75                                 M  P  P  P   F    E-2  AR-150   5.00                     CO-436   3.75                                 P  P  P  P/M P    E-3  AR-150   5.00                     CO-436   2.52                                 P  P  P  F   P    C-1  AR-150   5.00                     CO-436   2.75                                 F  -- P  --  --    C-2  AR-150   7.50                     CO-436   2.52                                 M  -- P  --  --    E-4  AR-150   5.00                     CO-436   2.52                                 M  P  P  P   M    E-5  AR-150   7.50                     CO-436   3.75                                 M  M  P  F   M    E-6  AR-150   5.00                     CO-436   2.52                                 P  P  P  F   P    E-7  AR-150   5.00                     CO-436   3.75                                 P  P  F  F   P    E-8  AR-150   1.25                     CO-436   3.50                                 P  P  P  F   P    C-3  Vinol    7.50                     CO-436   2.25                                 F  -- -- --  --    C-4  Vinol    10.00                     CO-436   3.00                                 M  -- F  --  --    C-5  Vinol    10.00                     CO-436   3.00                                 M  -- F  --  --    E-9       (a)         AR-150   5.00                     CO-436   3.80                                 P  P  P  F   P       (b)         AR-150   5.00                     CO-436   3.60    E-10       (a)         AR-150   5.00                     CO-436   2.66                                 P  P  P  M   P       (b)         AR-150   5.00                     CO-436   2.52    C-6       (a)         AR-150   5.00                     CO-436   1.90                                 M  P  P   M+ F       (b)         AR-150   5.00                     CO-436   1.80    C-7       (a)         Triton X165                  5.00                     CO-436   3.80                                 M  M  -- M   --       (b)         Triton X165                  5.00                     CO-436   3.60    E-11       (a)         Triton X165                  5.00                     CO-436   3.80                                 P  M  -- M   --       (b)         Triton X165                  5.00                     CO-436   3.60    C-8       (a)         None     0.00                     CO-436   3.80                                 F  -- -- --  --       (b)         None     0.00                     CO-436   3.60    C-9       (a)         None     0.00                     CO-436   3.80                                 F  -- -- --  --       (b)         None     0.00                     CO-436   3.60    C-10       (a)         AR-150   5.00                     CO-436   3.75                                 -- P  P  P   F       (b)         AR-150   7.50                     CO-436   2.52    E-12       (a)         AR-150   5.00                     CO-436   3.75                                 -- M  P  P   P       (b)         AR-150   7.50                     CO-436   2.52    E-13       (a)         AR-150   5.00                     CO-436   3.75                                 -- M  P  F   P       (b)         AR-150   7.50                     CO-436   2.52    E-14       (a)         AR-150   5.00                     CO-436   3.75                                 -- M  P  F   P       (b)         AR-150   7.50                     CO-436   2.52    __________________________________________________________________________

The chain transfer agent level is also used to ensure repulpabilitywhile maintaining adhesive performance. An excess of the amount of chaintransfer agent results in polymers with poor adhesive properties. Toolittle chain transfer agent results in an inability to repulp theadhesive polymer. Increasing the amount of chain transfer agent reducesthe polymer molecular weight and conversely decreasing the amount ofchain transfer agent increases the polymer molecular weight. One couldalso increase or decrease polymer molecular weight by other means suchas changing polymerization temperature to achieve adequatedispersibility.

On the average, the amount of n-dodecyl mercaptan found to be mostfunctional is about 1%±0.5% by weight based on the total weight of themonomers.

For Examples E-12 to E-14 and Control 10 blends of polymers were used,each polymer being polymerized in a manner used for individualpolymerization. The individual polymers were blended in a proportion of50% to 95% (a) polymer and 50 to 5% (b) polymer. As indicated by TableIV the presence of dodecyl mercaptan at about 0.5% for a single batchpolymerization was marginal as to utility of the product and at 1.5%marginal with respect to utility as to bleed. To our surprise, however,two stage sequential monomers addition provided a more aggressiveadhesive than a blend of polymers and allowed a reduction of n-dodecylmercaptan to achieve the same overall adhesive performance.

Using a sequential polymerized procedure as generally set forth above,the polymers of the Example E-15 to E-19 were prepared with the relativeproportions of monomer and polymer properties set forth in Table V.

                  TABLE V    ______________________________________    Examples            E-15     E-16     E-17   E-18   E-19    ______________________________________    Sequential            yes      yes      yes    yes    yes    Polymer-    ization    Monomers    Ratio 1st            2/3      2/3      2/3    2/3    2/3    stage/2nd    stage    1st Stage,    Wt %    Monomer    2-EHA   60       60       60     60     72.4    VAc     12.5     12.5     12.5   12.5   13.7    MA      12.5     12.5     12.5   12.5   13.7    AA      11.25    11.25    11.25  11.25    MAA     3.75     3.75     3.75   3.75    DAM     0        0        0      0      0.2    n-DDM   0        0        0      0      0    2nd Stage,    Wt %    Monomer    2-EHA   60       60       60     60     34.4    VAc     12.5     12.5     12.5   12.5   27.9    MA      12.5     12.5     12.5   12.5   27.9    AA      15       15       15     15     7.8    MAA     0        0        0      0      2.0    n-DDM   0.3      0.3      0.66   0.83   2.2    Cold Water            good     good     good   good   good    Wash-off    Cardboard            good     good     good   good   marginal    Adhesion    Shear 500 g            71 min   180 min  142 min                                     68 min 14 min    wt  1/4 sin            c/p      c/p      c/p    2 mil    Mylar film    2 mil    Mylar film    90% RH,          marginal marginal                                     marginal    40° C.    TAPPI   pass     pass     pass   pass   pass    UM 204    Comments            rxn temp rxn temp rxn temp                                     rxn temp                                            rxn temp            50-55° C.                     68-72° C.                              68-72° C.                                     68-72° C.                                            68-72° C.    Solids (%)            50    Viscosity            7000    (cP)    ______________________________________     rxn = polymerization temperature

The repulpable pressure-sensitive adhesives of the instant invention areecologically friendly and are adaptive for use with a variety of paperbacking or stock including paper stock useful in the postal industrysuch as papers used for stamps, labels, envelopes and the like. Suchpapers include alkaline-processed papers, acid-processed papers andprephospherized-acid-processed papers.

When the adhesives are combined with a paper face stock paper used forstamps the adhesive will be applied over a water soluble layer whichallows separation of the stamp face stock from the adhesive andproviding on the opposed surface with means to enable identification ofthe location of a stamp on a properly stamped envelope to enable anautomatic cancellation machine to determine if the envelope can beautomatically cancelled or rejected for hand cancellation.

The preferred means of identification of a stamp is a zinc orthosilicate(Taggant) contained in the paper or applied as coating thereto. Theunder surface of the face stock is coated with a water soluble layerwhich allows through soaking for a period of no more than 30 minutesseparation of the stamp face from the adhesive for stamp collectorpurposes. The next layer in the construction is the pressure-sensitiveadhesive of the instant invention applied as an acrylic emulsion.

The pressure-sensitive adhesive layer is in contact with a releasesurface provided by a release liner or the opposed surface of the paperface stock. If provided by the opposed surface of the face stock and therelease used must accept cancellation inks.

In respect to the compositions of the instant invention, some papermills regard the Tappi test as too stringent and compositions which mayfail the Tappi test may still be regarded as repulpable adhesiveprovided they do not accumulate in the recovered paper fiber Pulp

During the course of our work, we found the acrylic emulsion adhesivesof greater than 20% total acid were water-dispersible, but notinherently tacky. However, through the addition of known plasticizingand tackifying agents adhesion (looptack peel on cardboard) can beimproved. These adhesives were not humidity tolerant, however and bledinto 50 #EDP facestock.

At acid levels below 10%, the adhesives were inherently more tacky andshowed good humidity performance i.e. no bleed under identical storageconditions. However, these systems were not repulpable by Tappi UM213methodology.

Blending the polymers at (MAA/AA 20% and 10%) at polymer ratios of 80:20to 95:5 gave products that marginally passed Tappi UM 213 with improvedlooptack adhesion to cardboard and improved bleed performance under highhumidity exposure.

Individual polymers which do not pass Tappi UM 213 may pass Tappi UM 204and are useful in markets where hot alkali repulping is employed, alsopolymers When blended with a polymer which passes UM213 many in thecombination pass UM 213.

For instance, the sequentially polymerized E-8 above passed Tappi 204and exhibits acceptable high-humidity bleed resistance on 50#EDP paper.Similarly, the blends of polymers E-10 (a) and (b) in proportion 80:20,when further blended with a polymer of 56.8% (weight basis) 2-EHA, 37.9%BA, 2.8% MAA, 1.9% AA, and 0.6% itaconic acid and formed in the absenceof a chain transfer agent ("Polymer B" herein) when blended inproportion of 80-20, 70-30 and 60-40, pass Tappi UM204 and high-humiditybleed at 25° C. (50#EDP) and are part of this invention.

For example certain blends of the polymer of Example E-16 and polymer Bgave the results shown in Table VI.

                  TABLE VI    ______________________________________              Wt % E-     Wash-off    1 Week    Paper     16/Polymer B                          (Repulp)    Aging.sup.2    ______________________________________    50#EDP    60/40       Good        Good Grab    Alkaline Pro-    cessed Virgin,    White    Consolidated              60/40       Fair at 40° C.                                      Good Grab    Acid    Processed             Good at 70° C.    Yellowish    Prephosphor-    ized    Gladfelter              60/40       Poor at 40° C.                                      No Grab Left    Alkaline    Processed             Fair at 70° C.    Prephosphor-    ized White    ______________________________________     .sup.2 At 90% Relative Humidity, 40° C.

The presently preferred composition of this invention is one containingabout 85.5% by weight of the sequentially polymerized polymer of E-18,about 4.5% by weight Polymer B tackified with about 10% by weightAquatack 6025 a rosin ester manufactured by Arizona Chemicals.

The product is fully dispersible at 100° F. and pH 8.5 which are verymild pulping conditions for paper mills. Addition of Polymer B resistsbleed at 40° C. and 100% relation humidity for about 2 weeks and thetackifier improved adhesive properties. The use of vinyl acetate andmethyl acrylate in equal amounts gave good cardboard adhesion, improvedrepulpability and reduced cost.

Surprisingly, externally crosslinking the blended formulations withAl(OH)₂ Ac. 1/3H₃ BO₃ greatly improved repulpability. Even marginallyrepulpable systems (like the 80:20 blend) become fully water dispersiblewhen cross-linked.

We found that adhesive polymers formed by sequential polymerization wereinherently more tacky than uncrosslinked and crosslinked polymers of thesame monomer composition prepared individually or blended.

By increasing base weight of the repulpable paper to 60 pounds per reamor using coated repulpable paper such as 55 pound per ream paper stocksuch as 110 paper by Repap, bleed is reduced and more adhesion retainedon exposure to high humidity.

What is claimed is:
 1. A construction comprising a repulpable paperbacking supporting on at least one surface thereof a water-dispersible,inherently tacky pressure-sensitive adhesive comprising a tacky emulsionpolymer formed from about 55% to 90% by weight of a first monomer whichis at least one alkyl acrylate containing from 4 to about 8 carbon atomsin the alkyl group, from about 10% to about 20% by weight of a mixtureof acrylic acid with at least one oleophilic unsaturated carboxylicacid, from 0% to about 15% by weight of at least one vinyl ester, andfrom 0% to about 15% by weight methyl acrylate, said emulsion polymerbeing formed in the presence of anionic and nonionic surfactants, saidpolymer having a glass transition temperature of about -15° to about-50° C., and formed in the presence of a sufficient amount of chaintransfer agent to provide an emulsion polymer which, when coated onto arepulpable paper substrate, is sufficiently dispersible to enablerecovery of paper fibers substantially free of adhesive and capable ofpassing TAPPI Useful Method 204, said adhesive being resistant to lossof adhesive properties and bleed on exposure to high humidity.
 2. Aconstruction comprising a repulpable paper backing supporting on atleast one surface thereof a water-dispersible, inherently tackypressure-sensitive adhesive, said pressure sensitive adhesive comprisinga mixture of:(a) about 85.5% by weight of a polymer formed by sequentialemulsion polymerization and containing on a weight basis, about 60% byweight 2-ethylhexyl acrylate, about 12.5% by weight vinyl acetate, about12.5% by weight methylacrylate, about 13.5% by weight acrylic acid, andabout 1.5% by weight methacrylic acid; (b) about 4.5% by weight of anemulsion polymer containing about on a weight basis, 56.8% 2-ethylhexylacrylate, about 37.8% by weight butyl acrylate, about 2.8% by weightmethacrylic acid, about 1.9% by weight acrylic acid, and about 0.6% byweight itaconic acid; and (c) about 10% by weight of a rosin estertackifier; said adhesive resisting bleed into paper and capable ofpassing TAPPI Useful Method
 204. 3. A construction as claimed in claim 1in which the repulpable paper backing is selected from the groupconsisting of alkaline-processed papers, acid-processed papers andprephosphorized acid-processed paper.
 4. A construction as claimed inclaim 3 in which the adhesive is also in contact with a release surface.5. A construction as claimed in claim 4 in which the release surface isprovided by a release liner.
 6. A construction as claimed in claim 4 inwhich the release surface is on a side of the paper backing opposed tothe side supporting the pressure-sensitive adhesive.
 7. A constructionas claimed in claim 3 in which a water soluble layer is containedbetween the pressure-sensitive adhesive and the repulpable layer, saidwater soluble layer enabling intact removal of the repulpable paper fromthe pressure-sensitive adhesive within about 30 seconds after immersionin water.
 8. A construction as claimed in claim 2 in which therepulpable paper backing which is selected from the group consisting ofalkaline-processed papers, acid-processed papers and prephosphorizedacid-processed paper.
 9. A construction as claimed in claim 8 in whichthe adhesive is also in contact with a release surface.
 10. Aconstruction as claimed in claim 9 in which the release surface isprovided by a release liner.
 11. A construction as claimed in claim 8 inwhich the release surface is on a side of the paper backing opposed tothe side supporting the pressure-sensitive adhesive.
 12. A constructionas claimed in claim 8 in which a water soluble layer is containedbetween the pressure-sensitive adhesive and the repulpable layer saidwater soluble layer enabling intact removal of the repulpable paper fromthe pressure-sensitive adhesive within about 30 seconds after immersionin water.
 13. A construction as claimed in claim 1 in which the acrylicacid is present in an amount of about 5% to about 19% by weight of thepolymer, and the oleophilic unsaturated carboxylic acid is present in anamount of from about 1% to about 15% by weight of the polymer.
 14. Aconstruction as claimed in claim 13 in which the weight ratio of acrylicacid to oleophilic unsaturated carboxylic acid is 2:3 to 7:1.
 15. Aconstruction as claimed in claim 1 in which the oleophilic unsaturatedcarboxylic acid is methacrylic acid.
 16. A construction as claimed inclaim 13 in which the oleophilic unsaturated carboxylic acid ismethacrylic acid.
 17. A construction as claimed in claim 14 in which theoleophilic unsaturated carboxylic acid is methacrylic acid.
 18. Aconstruction as claimed in claim 15 in which the oleophitic unsaturatedcarboxylic acid is methacrylic acid.
 19. A construction as claimed inclaim 15 in which the oleophilic acid is methacrylic acid in which theacrylic acid is present in an amount of from about 30% to 60% by weightof the mixture of acrylic acid and methacrylic acid.
 20. A constructionas claimed in claim 1 in which the first monomer is a mixture of a butylacrylate with a first alkyl selected from the group consisting of2-ethylhexyl acrylate, isooctyl acrylate and mixtures thereof.
 21. Aconstruction as claimed in claim 13 in which the weight ratio of firstacrylate to butyl acrylate is about 3 to
 1. 22. A construction asclaimed in claim 21 in which the oleophilic acid is methacrylic acid inwhich the acrylic acid is present in an amount of from about 30% toabout 60% by weight of the mixture of acrylic acid and methacrylic acid.23. A construction as claimed in claim 1 in which the nonionicsurfactant is an ethoxylated rosin acid.
 24. A construction as claimedin claim 1 in which the anionic surfactant is a sulfated ethoxylatednonylphenol.
 25. A construction as claimed in claim 23 in which theanionic surfactant is a sulfated ethoxylated nonylphenol.
 26. Aconstruction as claimed in claim 1 in which the polymer is formed bysequential polymerization by addition to a pre-emulsion containing about40% to about 80% of the total monomers and followed by addition of apre-emulsion containing the balance of the monomers.
 27. A constructionas claimed in claim 1 in which the polymer is formed by sequentialpolymerization in an aqueous medium of a first monomer pre-emulsioncontaining about 80% of the total monomers and about 20% by weight ofthe mixture of acrylic acid and the oleophilic unsaturated carboxylicacid and nonionic surfactant, followed by addition of a second monomerpre-emulsion containing the balance of the monomers and about 10% byweight of the mixture of acrylic acid and the oleophilic unsaturatedcarboxylic acid and anionic emulsifier.
 28. A pressure-sensitiveadhesive as claimed in claim 1 in which the emulsion polymer is formedof a blend of polymers.